1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to sensors for collecting seismic or similar data and, more particularly, to mechanisms and techniques for correcting a temperature-induced phase change of a recording signal during a seismic survey.
2. Discussion of the Background
Seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (either on land or seabed). While this profile does not provide an exact location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface of the earth is important, for example, to those who need to determine where oil and gas reservoirs are located.
Traditionally, a land seismic survey system 10 is generally configured as illustrated in FIG. 1, although many other configurations are used. System 10 includes plural receivers 12 and acquisition units 12a positioned over an area 13 of a subsurface to be explored and in contact with the surface 14 of the ground. A number of vibroseismic or other types of sources 16 are also placed on surface 14 in an area 17, in a vicinity of area 13 of receivers 12. A recording device 18 is connected to a plurality of receivers 12 and placed, for example, in a station-truck 20. Each source 16 may be composed of a variable number of vibrators or explosive devices, typically between 1 and 5, and may include a local controller 22. A central controller 24 may be present to coordinate the shooting times of the sources 16. A GPS system 26 may be used to time-correlate sources 16 and receivers 12 and/or acquisition units 12a. 
With this configuration, sources 16 are controlled to generate seismic waves, and the plurality of receivers 12 record waves reflected by oil and/or gas reservoirs and other structures. The seismic survey may last for days, or it may be repeated at various time intervals, e.g., months or years apart, to determine the original shape or changes in the monitored reservoirs. Either way, daily temperature changes occur during the life of the seismic survey, and they negatively impact the quality of the recorded signals as now discussed.
Receiver 12 may be a geophone having a structure as illustrated in FIG. 2. This structure may be found in other transducers that transform mechanical energy into electrical energy, i.e., seismometer, or any other device that uses a magnet field for energy conversion. For simplicity, this application refers herein to a geophone, but the novel operation principle described herein also applies for other types of transducers. Geophone 200 has a casing 202 that houses a magnet 204 and a coil assembly 206. Coil assembly 206 is electrically connected to terminals 210, and these terminals may be electrically connected to an optional damping resistor 208. When in use, the casing 202 moves in response to seismic waves in the earth. Either the coil assembly 206 or the magnet 204 is attached to the casing 202 and the mass of the other is suspended in such a way as to cause relative movement between the coil assembly 206 and the magnet 204. This movement of coil assembly 206 relative to magnet 204 produces a back electromagnetic force (emf) in the wires of the coil assembly. When damping resistor 208 is present, the back emf generates a current through terminals 210 and resistor 208. Voltage across terminals 210 is measured and considered to be indicative of the seismic signal to be recorded. Thus, a signal (having an amplitude and a phase) is recorded by the geophone 200 based on the voltage across optional damping resistor 208.
However, the following problem affects recorded signal quality. When ambient temperature of the geophone changes, for example, from 10° C. during the night to more than 40° C. during the day, magnetic properties of the magnet change. Depending on the type of magnet, its field can change more or less with the temperature. The cheaper the magnet, the more significant the magnetic field change. In fact, all magnets, irrespective of their price, experience a magnetic field change with changing ambient temperature.
The change in the magnetic field degrades the quality of recorded seismic signals because a change in the magnetic field determines a change in the electromagnetic damping of the geophone. The concept of electromagnetic damping is now explained. If no damping exists, after casing 200 moves due to seismic waves, coil assembly 206 will move in an oscillatory fashion until its energy is dissipated, e.g., through friction. These oscillatory movements of a second-order system are well-known to someone skilled in the art. The residual oscillations are undesirable because they interfere with recording of later ground motion. Therefore, damping is provided to the coil assembly to limit the number of oscillations, if possible, to one. Two types of damping are present in a geophone. Intrinsic damping is provided by eddy currents that appear in the coil support mechanism, which in turn generate a magnetic field opposing the magnetic field generated by magnet 204. A second type of damping may be provided by adding damping resistor 208, which has a resistance calculated in such a way to optimize the assembly motion.
The calculated value of the damping resistor 208 is only effective for a given temperature. If the ambient temperature of the geophone changes as noted above, there can be no unique match between a fixed resistor and the magnetic field temperature profile. Thus, even with a precision damping resistor 208, the geophone is prone to introducing phase errors into the measured signal when the temperature deviates from the nominal temperature for which the resistance of the damping resistor has been calculated.
Given that for a land seismic survey some geophones may be in full sun and some may be in complete shade, phase errors introduced by the geophones because of variations in their magnetic properties may be significant. For example, it has been observed that two identical geophones simultaneously recording seismic waves, one in shade and one in full sun, recorded the corresponding signals with a phase difference of up to 20 degrees. This large phase difference in recorded signals introduces inaccuracies when the central recording unit or other processing units add together several recorded signals in a process commonly called stacking or utilizes other mathematical processes.
One way to alleviate this problem is to develop magnets that are not temperature-sensitive, i.e., magnets with low thermal coefficients. However, such magnets are expensive and may have their own limitations because any magnet is, to a certain degree, temperature-sensitive. Further, these low-thermal-coefficient magnets have geometric limitations.
Thus, there is a need to develop technologies and methods for compensating temperature-induced magnetic field changes so that the geophones, irrespective of the ambient temperature, record accurate phases of the seismic signals.